The United States said on Friday it was imposing sanctions and cutting diplomatic ties with Libya as Muammar Gadhafi's security forces stepped up efforts to crush a widening revolt against his rule.

White House spokesman Jay Carney said the United States was suspending embassy operations in Tripoli and would withdraw all staff. Washington was also moving ahead with unilateral sanctions, which would be coordinated with allies and others.

Carney said Gadhafi's legitimacy had been "reduced to zero" and he had lost the confidence of the Libyan people.

Carney said the sanctions would be finalized in the near future but he did not specify when or what restrictive measures would be imposed.

"We are initiating a series of steps at the unilateral level and multilateral level to pressure the regime in Libya to stop killing its own people," he said, after Libyan government forces shot dead protesters in clashes in the capital Tripoli.

The Obama administration said earlier it is studying options that include sanctions, asset freezes, a "no-fly" zone over Libya and military action.

Carney was speaking after a chartered ferry and a plane carrying Americans and others left Tripoli. Fears for the safety of the Americans had tempered President Barack Obama's response to the crisis, forcing him to adopt a more cautious approach until U.S. citizens were out of harm's way.

After talking by phone on Thursday with the leaders of Britain, France and Italy on immediate steps to end the crisis, Obama spoke with Turkish Prime Minister Tayyip Erdogan on Friday, the White House said.

In a first step, the U.S. Treasury has told American banks to closely monitor transactions that may be related to unrest in Libya for any possible signs that state assets were being misappropriated.

The Treasury advisory, issued late on Thursday, stopped short of freezing assets or imposing other financial sanctions on Gadhafi or other senior officials in his government.

The move followed Switzerland's announcement on Thursday that it was freezing any assets Gadhafi and his family might have in the country.